Inkjet printing mechanisms use cartridges, often called "pens," which eject drops of liquid colorant, referred to generally herein as "ink," onto a page. Each pen has a printhead formed with very small nozzles through which the ink drops are fired. To print an image, the printhead is propelled back and forth across the page, ejecting drops of ink in a desired pattern as it moves. The particular ink ejection mechanism within the printhead may take on a variety of different forms known to those skilled in the art, such as those using piezo-electric or thermal printhead technology. For instance, two earlier thermal ink ejection mechanisms are shown in U.S. Pat. Nos. 5,278,584 and 4,683,481. In a thermal system, a barrier layer containing ink channels and vaporization chambers is located between a nozzle orifice plate and a substrate layer. This substrate layer typically contains linear arrays of heater elements, such as resistors, which are energized to heat ink within the vaporization chambers. Upon heating, an ink droplet is ejected from a nozzle associated with the energized resistor. By selectively energizing the resistors as the printhead moves across the page, the ink is expelled in a pattern on the print media to form a desired image (e.g., picture, chart or text).
To clean and protect the printhead, typically a "service station" mechanism is supported by the printer chassis so the printhead can be moved over the station for maintenance. For storage, or during non-printing periods, the service stations usually include a capping system which substantially seals the printhead nozzles from contaminants and drying. Some caps are also designed to facilitate priming, such as by being connected to a pumping unit that draws a vacuum on the printhead. During operation, clogs in the printhead are periodically cleared by firing a number of drops of ink through each of the nozzles in a process known as "spitting," with the waste ink being collected in a "spittoon" reservoir portion of the service station. After spitting, uncapping, or occasionally during printing, most service stations have an elastomeric wiper that wipes the printhead surface to remove ink residue, as well as any paper dust or other debris that has collected on the printhead. The wiping action is usually achieved through relative motion of the printhead and wiper, for instance by moving the printhead across the wiper, by moving the wiper across the printhead, or by moving both the printhead and the wiper.
As the inkjet industry investigates new printhead designs, the tendency is toward using permanent or semi-permanent printheads in what is known in the industry as an "off-axis" printer. In an off-axis system, the printheads carry only a small ink supply across the printzone, with this supply being replenished through tubing that delivers ink from an "off-axis" stationary reservoir placed at a remote stationary location within the printer. Narrower printheads may lead to a narrower printing mechanism, which has a smaller "footprint," so less desktop space is needed to house the printing mechanism during use. Narrower printheads are usually smaller and lighter, so smaller carriages, bearings, and drive motors may be used, leading to a more economical printing unit for consumers.
To improve the clarity and contrast of the printed image, recent research has focused on improving the ink itself. To provide quicker, more waterfast printing with darker blacks and more vivid colors, pigment-based inks have been developed. These pigment-based inks have a higher solid content than the earlier dye-based inks, which results in a higher optical density for the new inks. Both types of ink dry quickly, which allows inkjet printing mechanisms to form high quality images on readily available and economical plain paper, as well as on recently developed specialty coated papers, transparencies, fabric and other media. However, the combination of small nozzles and quick-drying ink leaves the printheads susceptible to clogging, not only from dried ink or minute dust particles, such as paper fibers, but also from the solids within the new inks themselves.
To clear clogged nozzles, frequent spitting routines are performed before, during, and after a print job. Unfortunately, the spitting operation generates inkjet aerosol, small minute ink particles or satellites which become detached from the main ink droplet and begin floating through the printer. These floating inkjet aerosol satellites may be carried by air currents flowing through the printer to land in undesirable locations. Often the inkjet aerosol lands on critical components inside the printer casing, for instance, resulting in fogging of the optical encoder used in carriage position control, or fouling portions of the casing and carriage where an operator would touch when installing a new pen. Sometimes this aerosol is deposited in the media path through the printer and then picked up by the next sheet of print media, leading to print quality defects.
While some inkjet aerosol maybe generated during a normal printing operation, the effect of this aerosol is not as severe as that generated during the spitting operation because during printing, the media is closer to the printhead than the typical spittoon target area is during spitting. For instance, when ink droplets are ejected to form images on media, the printhead is usually spaced about one millimeter (1 mm) above the media. In contrast, when ink droplets are ejected during a spitting routine, the vertical distance between the printhead orifice plate and the spittoon target surface is usually greater than five millimeters (&gt;5 mm). Since there is a tendency sometimes for the ejected droplets to shoot at an angle other than 90.degree. from the orifice plate, referred to as a misdirected droplet, a larger distance between the orifice plate and the target leads to a greater drop trajectory error. Thus, it would be desirable to have a spit target which is large enough to collect any misdirected ink droplets. Moreover, this greater distance which a droplet must travel before impacting the spit target gives the droplet, and any associated inkjet aerosol, a greater chance to drift away from the intended spit target, due to the air currents flowing within the printer and due to electrostatic charges on the droplets, aerosol satellites, and surrounding printer components. While a simple solution may appear to be just merely making the spittoon target area larger, this impacts other printer design constraints, such as the desire to provide a compact printer with a small footprint which occupies a minimal amount of desktop or workspace.
In the past, several different approaches have been used to control inkjet aerosol, including modified spittoons, absorbers, and fans. First, regarding spittoon design, spittoons are essentially large buckets over which the pens are parked when droplets are ejected during a spitting routine. Unfortunately, spittoon design constraints often restrict the top of the bucket from being close enough to the pen face to limit the spread of the droplets caused by trajectory errors, air currents, electrostatic charges, etc. Moreover, the opening at the top of the bucket must be sized large enough so most of the droplets reach the bottom of the bucket, rather than impacting the bucket sides. Droplets hitting the sides of the bucket often dry there, and in some instances have eventually formed a solid ink bridge across the bucket. Such an ink residue bridge greatly decreases the capacity of the bucket because ink residue then builds up from the bridge, rather than from the bottom of the bucket, until in a worst case scenario the residue reaches the pen face, most likely leading to a pen failure. The combined effects of the restricted size of the top of the bucket and its location away from the pen face often result in some of the ink droplets and aerosol being captured by internal air currents and carried away for deposit in undesirable locations.
The second manner of controlling ink aerosol involves using various absorbers. These absorbers are usually made of some type of a fiber, such as a felt, sponge, or other type of porous material which lines the bottom of the spittoon. Using these absorbers, droplets of ink are typically wicked through capillary forces from the top of the bucket toward the bottom of the bucket. This wicking action prevents the bridging of ink residue across the spittoon. Unfortunately, these absorbers often need to be spaced five millimeters (5 mm) or more from the pen orifice plate, often to prevent loose fibers on the surface of the absorber from contacting the printhead, or due to tolerance issues stemming from the material composition or the fabrication techniques used to make the absorber. For instance, if the absorber is formed through a die-cutting process, any irregularities in the die may lead to uneven cuts, which may leave portions of the absorber projecting into the printhead path if a closer pen-to-absorber spacing was used. Moreover, the width of the absorber is often limited by the space allocated within the printer, so without impacting the printer footprint, the absorber cannot be made large enough to compensate for worst case drop trajectory errors which exacerbated by the larger absorber-to-orifice-plate distances. Thus, typical absorbers also fall short of controlling inkjet aerosol due to these various design, material and manufacturing constraints.
A third way to control inkjet aerosol has been through the use of forced ventilation provided by one or more fans. Ventilation fans have been a powerful inkjet et aerosol control technique, essentially creating air currents that pull the aerosol through the printer. As the air stream flows through the printer, the floating aerosol satellites are entrained within the air stream, which is then forced through a filter to remove the aerosol particles. Such an aerosol controlling fan and filter assembly was first used on the Hewlett-Packard Company's model 850C DeskJet.RTM. color inkjet printer. Unfortunately, while the fan and filter assembly performed very well, it increased both the overall initial cost to consumers, and operating costs from electricity consumed by the fan.
Thus, it would be desirable to have spittoon system which captures ink aerosol and misdirected ink droplets generated during a spitting routine before these droplets and aerosol satellites float away to land at other undesirable locations.